The conversion and utilization of carbon dioxide becomes still more important in view of its environmental significance. Electrochemical reduction of CO2 provides a potential renewable route to carbon-based fuels. Largely investigated has been the electrochemical reduction of CO2 in aqueous solutions, methanol and some organic aprotic solvents. The effect of the nature of electrolytic medium, electrode material and concentration of CO2 on the Faraday efficiency has also been reported. Numerous catalysts have been reported for the electrochemical reduction of CO2 and the products of the catalytic reduction include oxalate, CO, formate, carboxylic acids, formaldehyde, acetone, methanol, methane and ethylene.
Although water is an environmentally clean medium, its use is limited due to the low solubility of CO2, the variety of products obtained during the reduction and the difficulty of products recovery. Using a cobalt porphyrin attached to glassy carbon electrode as catalyst for CO2 reduction, the electrode was active for the electroreduction of CO2 to CO and H2 in aqueous medium with a current efficiency of CO production of 92% at −1.1 V [1]. Another alternative is the use of organic solvents, however this is prohibitive due to their toxic and hazardous nature. It has been reported that CO2 can also be reduced in molten eutectic mixture of Li2CO3+Na2CO3+K2CO3 at 700° C. [2]. This medium allowed high solubility of CO2 (˜0.1 M). However, the current densities obtained for the reduction of CO2 were very low. This was explained as being due to a reaction occurring between CO2 and carbonate ions to yield C2O52− ions which are difficult to reduce. The reduction of CO2 to O2 and CO in the 400-700° C. temperature range with a ceramic electrolyte has also been reported [3].
Ionic liquids are salts which are in the molten state at low temperatures (<100° C.); they are considered to be green solvents due to their very low vapor pressure and chemical inertness. High conductivity and wide electrochemical windows make them very useful electrolytes with wide potential applications. Ionic liquids were suggested for use as an electrolyte for the reduction of CO2 [4]. Although the solubility of this gas is high in these solvents, supercritical CO2 was supplied to the cathode, and when water was added the ionic liquid, CO and H2 were obtained at the cathode and O2 at the anode. A known method to overcome mass limitations of gases being reduced (such as O2 in fuel cells) is by the use of gas diffusion electrodes which interface the gas, electrocatalyst and electrolyte phases. However, when a liquid electrolyte is used, the pores of the electrode at which the gas is reduced are prone to flooding. This can be overcome by using a solid polymer electrolyte, such as the perfluorosulfonate membranes (such as Nafion) used in fuel cells. This membrane has also been used for the electrochemical reduction of CO2 to CH4 and C2H4 [5, 6]. However, this membrane functions only in strong acidic media and very small faradaic efficiencies have been achieved for the reduction of CO2 at gas diffusion electrodes [5,6]. It is therefore an object of this invention to provide a method for reducing CO2 at gas diffusion electrodes with a gel or solid electrolyte comprising an ionic liquid, while avoiding the drawbacks of the previous techniques.
It is further an object of the invention to provide a method for reducing CO2 at gas diffusion electrodes with an ionic liquid, trapped in a gel or membrane which serves as electrolyte. Besides the benefit of being environment friendly, these matrices will allow high CO2 solubility, and relatively high conductivity even at low water content.
It is another object of this invention to provide an electrochemical cell comprising an anode and a cathode, and an electrolyte in the form of gel or membrane comprising an ionic liquid, for use in manufacturing carbon-based combustibles.
Other objects and advantages of present invention will appear as description proceeds.